i'60 



NEW YORK STATE MUSEUM 



or Tupper lake. Its minerals are hornblende, augite, bronzite, 

 feldspars and quartz, with accessory magnetite, apatite and zir- 

 con, and garnet also in the outer parts of the mass. The feld- 

 spar is mostly anorthoclase, some showing plainly irregular inter- 

 growths of two feldspars, probably orthoclase and albiie. Quite 

 a little oligoclase is present in addition. Quartz constitutes some 

 15;/ of the rock. 



The outcrops of this rock are not extensive, and it clearly is a 

 mass of no great size, extending only a few rods in any direction. 

 On the east, down the hill, the rock appears to be grading into 

 the ordinary gneiss, but no further exposures appear in that 

 direction. In all other directions anorthosite-gabbro or else 

 gneiss is reached at no great distance, but there are absolutely no 

 contacts showing anywhere, so that the field relations of this ap- 

 parent syenite are left in entire uncertainty. So far as the age 

 of the augite-syenite is known, it is younger than the anorthosite, 

 but that need not necessarily imply that it is younger than this 

 anorthosite-gabbrO. This occurrence may be a dike, though that 

 is unlikely, as its extension could not anywhere be found; it may 

 . be a small intrusive mass, though it is exceptionally small for a 

 mass of such origin. In the utter absence of field evidence the 

 reference is made simply on the character of the rock. 



Later igneous rocks 



The much later age of these rocks is inferred from 

 the fact that they are wholly unmetamorphosed, though 

 they have suffered deformation. It is thought that this 

 deformation would have caused metamorphism, had the rocks 

 been buried deeply enough. The inclosing rocks are older 

 and were metamorphosed before these later rocks found their way 

 through them. When metamorphosed, they were deeply buried 

 in the zone of flow, and are now exposed at the surface because 

 all the overlying rock has been eroded away. But the later rocks 

 exposed at the surface have never been in that zone. Much less 

 rock had to be worn away to bring the present exposures to the 

 surface, and hence a. long erosion period between the time of 



